Mixer circuits, or mixers, are widely used in modern communication systems and have a number of uses. A typical application is for translating signals into desirable frequency bands through a technique called modulation. In a mixing or modulation process, an information signal is superimposed upon a carrier signal. In this way, the information signal may be processed or transmitted while minimizing, and at times without, information loss. Another principle application of mixer circuits is for demodulation. In demodulation, a modulated information signal is typically translated to an intermediate frequency.
Mixers are generally designed to minimize inter-modulation (IM) distortion. This distortion is usually caused by non-linear translations of the input signal and typically affects the dynamic range of the communication system. Additionally, mixers are generally designed to sustain large interference signals without desensitizing while maintaining a low noise figure to optimize overall system performance.
Analog multipliers or mixers may be used in a wide range of communications applications. For example, analog multipliers are typically used in down converters in radio frequency (RF) receivers. In RF receivers, analog multipliers may be used to convert a high frequency or RF input signal to an intermediate frequency (IF) signal or to a base-band signal. To perform a down conversion, the analog multiplier receives a carrier RF input signal as well as a mixing frequency from a local oscillator (LO). A desired output signal from the down converter, usually the IF signal, is generally a difference of the RF input signal and the mixing frequency LO signal (e.g., RF−LO). In addition, other signals, including a signal at a center frequency of a sum of the RF input signal and the mixing frequency LO signal (e.g., RF+LO), as well as harmonics of fundamental frequencies, are usually generated. For a downconverter application, the frequency generated at (RF+LO) generally has the greatest amplitude of the undesired signals.
In low-IF or direct-conversion receiver architecture, the mixer output may be directly coupled to an analog base-band. Second-order inter-modulation (IM2) spurs can pass to the mixer output. The result of this may be a raised noise floor component of the mixer output. The IM2 spurs are generally caused by mismatches of devices in the receiver architecture which may also occur in differential circuit topology. In the event that all components of the receiver architecture are ideally matched, the second-order terms, derived from the input signal, at the output tend to cancel each other. In a mismatched design, the second-order terms tend to remain in the output.
One concern with low-IF or direction-conversion receiver architectures is a relatively stringent requirement for IIP2 and third-order input intercept point (IIP3). For a direct-conversion receiver where a desired signal of interest is downconverted from RF to base-band through a single modulation process, interference from second-order nonlinearities, such as IM2, is usually detrimental to system performance. To protect a direct-conversion receiver from such undesirable spurious response, a high second-order input intercept point (IIP2) is desirable. IIP2 generally represents an input amplitude at which the desired signal becomes equal in amplitude to a spectral component generated from IM2.
Accordingly, a mixer circuit is desired having high linearity and low noise that may be used in direct-conversion receivers or low-IF receivers. In addition, a mixer circuit is desired having low noise performance of a buffer stage, high IIP3 of a buffer stage, and high IIP2 of the switch stage. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.